The present invention relates to a microcomputer contained in an IC card or the like, and particularly to a clock signal for writing in EEPROM provided as a memory unit in a microcomputer.
Writing in EEPROM generally requires a clock signal in order to obtain synchronism, and writing of data etc. is performed synchronously with this write clock signal. Some conventional EEPROMs contain oscillating circuits for the write clock signal. The time spent writing in EEPROM depends upon the frequency of the clock signal output from such a oscillating circuit. FIG. 4 is a schematic block diagram of the configuration of a conventional microcomputer which contains EEPROM having an oscillating circuit and divider and which is incorporated in an IC card or the like. In FIG. 4, a microcomputer module 10 comprises a CPU 1 serving as a processing unit and EEPROM 2 serving as a fixed memory unit. The CPU 1 is shown as a unit having a processing function and containing an ALU, a register, a counter, memory (none of which are shown in the drawing) and the like which are required for processing. The CPU 1 delivers data to the outside thereof through a data input and output bus 3a and receives a clock signal from the outside through a clock signal line 3b. The EEPROM 2 contains a storage unit 2a comprising a memory array, a control circuit necessary for writing data in the memory array and a latch circuit (none of which are shown in the drawing), an oscillating circuit 2b for oscillating a clock signal with a natural frequency and a divider 2c for dividing the clock signal oscillated by the oscillating circuit 2b to convert the clock signal to a clock signal with a given frequency. The CPU 1 and the EEPROM 2 are connected by a control bus 4, an address bus 5 and a data bus 6. The clock signal divided by the divider 2c is sent to the storage unit 2a through a clock signal line 2d.
In this EEPROM 2, the clock signal generated by the oscillating circuit 2b contained therein is divided with a given ratio by the divider 2c so that data is written synchronously with the clock signal divided.
Although the conventional microcomputer is configured as described above, dispersion occurs during data writing because the clock signal generated by the oscillating circuit contained in EEPROM is generally unstable. It is therefore necessary to set a value in the divider so that the minimum value of dispersion of the writing time satisfies the writing time. There is thus the problem that it is necessary to set a longer time than the time required for the actual writing, resulting in a long write time.